Big mergers and acquisitions make for splashy headlines, but do they make financial and strategic sense? Executives, board members, and investors are wise to be skeptical. Such deals—worth 30 percent or more of the acquirer’s market capitalization—are extremely complex. And as high-profile failures have demonstrated, big deals can destroy significant value for shareholders.

Big deals can create significant value for the acquirer, however, even if success takes time to unfold. Indeed, in our analysis of such deals over the past decade, half had created excess returns to shareholders when measured two years after the deal’s completion. In one-third, returns were significantly higher relative to the industry average.

The difference between success and failure often comes down to strategy. Only a few situations give companies a clear, compelling reason to take on a big deal’s risks and integration complexity. Companies with few options for organic growth, for example, can use a large deal to enter a new sector or market quickly. Those in consolidated industries, such as oil and gas or mining, can find success in big deals when other options are limited and major economies of scale exist. And on the rare occasion when a large target company is a very clear strategic fit with the prospective buyer, a big deal can improve an acquirer’s growth and performance rapidly.

But a successful deal also results from strong execution. In case studies of nine of the best-performing deals and six of the worst in our dataset, we found that successful acquirers employ several approaches to execution and integration that are different from those used by unsuccessful ones—and different from those typically used by acquirers in smaller deals. Successful acquirers set performance targets higher than due-diligence estimates of a merger’s value. They reject the common idea that an acquisition represents an opportunity to adopt the best of two companies’ cultures. Finally, their CEOs focus their involvement on a few most critical areas.